CN113544525A

CN113544525A - Gradient coil system

Info

Publication number: CN113544525A
Application number: CN201980082373.9A
Authority: CN
Inventors: F·刘; R·魏; F·唐
Original assignee: Magnica Co ltd
Current assignee: Magnica Co ltd
Priority date: 2018-12-13
Filing date: 2019-11-20
Publication date: 2021-10-22
Also published as: JP2022512410A; EP3894879A4; US11422214B2; EP3894879B1; AU2019396124B2; EP3894879A1; WO2020118352A1; AU2019396124A1; EP3894879C0; US20220011387A1; JP7362147B2

Abstract

A gradient coil system suitable for use in an MRI system. The gradient coil system has a gradient body having an aperture extending therethrough and at least one frustoconical portion disposed about the aperture. The diameter of the first end of the bore is greater than the diameter of the second end of the bore. The gradient coil system also includes a gradient coil assembly disposed about the bore, the gradient coil assembly having at least one frustoconical portion substantially conforming to the at least one frustoconical portion of the gradient body, the gradient coil assembly generating a gradient field for medical imaging in a diameter of a spherical volume (DSV).

Description

Gradient coil system

The present invention relates to a gradient coil system for Magnetic Resonance Imaging (MRI) devices and arrangements. In particular, the invention relates to a frustoconical gradient coil system using shimming to improve imaging.

Background

Any reference to methods, apparatus or documents of the prior art should not be taken as any evidence or admission that they form or form part of the common general knowledge.

Magnetic resonance imaging was introduced in the 1980 s and has evolved into a major medical imaging modality.

The success of clinical MRI relies on the generation of strong and uniform magnetic fields. MRI machines are designed to generate a static magnetic field that requires substantial homogeneity over a predetermined region, referred to in the art as a "diameter spherical imaging volume" or "DSV". Deviations in static field uniformity across a DSV typically require less than 20 parts per million peak (or 1rms per million).

Since the introduction of the first closed cylinder system, MRI devices have undergone many improvements. In particular, improvements have been made in the quality/resolution of images by improving the signal-to-noise ratio and introducing high and ultra-high field magnets. The increase in image resolution in turn has led to MRI becoming an increasingly expert-selected modality for structural anatomical and functional human MRI imaging.

The basic components of a typical magnetic resonance system for producing diagnostic images of human studies include a main magnet, typically a superconducting magnet that produces a substantially uniform static magnetic field (B0 field) in a DSV, one or more sets of shim (shim) coils, a set of gradient coils, and one or more RF coils. For the discussion of MRI, one can use magnetic resonance imaging in, for example, Haacke et al: physical Principles and Sequence Design, John Wiley & Sons, Inc., New York,1999 (Magnetic Resonance Imaging: Physical Principles and Sequence Design, John Wiley & Sons, Inc., New York, 1999). See also, U.S. patent 5,818,319 to Crozier et al, U.S. patent 6,140,900 to Crozier et al, U.S. patent 6,700,468 to Crozier et al, U.S. patent 5,396,207 to Dorri et al, U.S. patent 5,416,415 to Dorri et al, U.S. patent 5,646,532 to Knuttel et al, and U.S. patent 5,801,609 to Laskaris et al, the contents of which are incorporated herein in their entirety.

Whole-body MRI magnets are generally cylindrical with a length of about 1.6-2.0 meters, with an axial bore in the range of 0.6-0.8 meters. Typically, the magnets are symmetrical such that the midpoint of the DSV is located at the geometric center of the magnet structure along its major axis. Not surprisingly, many people suffer from claustrophobia when placed in such spaces. Moreover, the large distance between the imaged part of the subject's body and the end of the magnet system means that a physician cannot easily assist or personally monitor the subject during the MRI procedure.

In addition to its effect on the subject, the size of the magnet is a major factor in determining the cost of an MRI machine and the cost of installing such a machine. Another important consideration is the volume of helium required to maintain the system at cryogenic temperatures. Due to their large size, such whole-body magnets are expensive to use for generating images of small-sized objects, such as the head, limbs, and neonates.

As mentioned above, gradient coils are a fundamental system component in Magnetic Resonance Imaging (MRI) scanners that provide spatial encoding of Nuclear Magnetic Resonance (NMR) signals during scanning. Strong and linear magnetic field gradients are required in fast imaging modalities such as diffusion weighted imaging and Echo Planar Imaging (EPI). The improvement in overall gradient strength can be achieved simply by adding more turns to the coil, however, this approach results in an increase in inductance and resistance.

The gradient strength can also be increased by using stronger gradient amplifiers, but this approach is not cost effective. The use of larger gradient power/current may also result in larger electric fields being induced in the human body and cause Peripheral Nerve Stimulation (PNS).

Another practical way to increase gradient strength is by applying local gradient coils, such as insertable head coils, within the whole-body MRI system. Insertable gradient coils have shown their advantages, such as enhanced gradient strength and minimized inductance, exhibiting high switching speeds and potentially higher PNS limits.

Geometrically, head gradient coils typically use an asymmetric configuration because the head access coil is limited by the size of the shoulder.

Accordingly, there is a need for an improved coil system for use in an MRI system.

Object

It is an object of the present invention to provide a gradient coil system which overcomes or ameliorates one or more of the disadvantages or problems described above, or which at least provides a useful commercial alternative.

Other preferred objects of the present invention will become apparent from the following description.

Disclosure of Invention

In a first aspect, the invention resides in a gradient coil system having one or more frustoconical portions, one or more cylindrical portions having different radii, or a combination of one or more frustoconical portions and one or more cylindrical portions.

In another aspect, the invention resides in a gradient coil system adapted for use in an MRI system, the gradient coil system comprising:

a gradient body having a bore extending therethrough and at least one frustoconical portion disposed about the bore, wherein a diameter of a first end of the bore is greater than a diameter of a second end of the bore; and

a gradient coil assembly having at least one frustoconical portion substantially conforming to the at least one frustoconical portion of the gradient body.

Preferably, in use, the diameter of the spherical volume (DSV) associated with the system is offset from the geometric centre of the system to allow easier imaging of both the head and the extremities.

In some embodiments, the high efficiency (i.e., high slew rate and gradient strength) of the frustoconical portion of the gradient coil assembly provides a higher maximum scan speed (i.e., fast imaging).

In some embodiments, the gradient body of the system comprises one (or a single) frustoconical portion extending along the length of the bore between the first end and the second end.

Preferably, the gradient body comprises at least one cylindrical portion arranged around the bore. Preferably, the cylindrical portion abuts the frustoconical portion. Alternatively or additionally, a first cylindrical portion having a diameter adjoins a second cylindrical portion having a diameter, wherein the diameter of the first cylindrical portion is larger than the diameter of the second cylindrical portion. Preferably, the plurality of frusto-conical portions and/or the cylindrical portion defines a stepped diameter bore.

In some embodiments, the gradient coil assembly includes a primary coil structure having at least one substantially frustoconical portion.

The gradient coil assembly may also include a shield layer structure having at least one substantially frustoconical portion.

Preferably, the gradient body is located within the chamber of the magnet.

Preferably, the primary coil structure comprises a first, a second and a third primary coil segment generating three orthogonal linear primary gradient fields in the DSV region. Preferably, the first primary coil segment comprises axial coils generating a first primary gradient field along the longitudinal axis (z-axis). Preferably, the second and third primary coil segments each comprise a transverse coil rotated 90 degrees relative to each other, thereby generating second and third primary gradient fields that are orthogonal to each other and to the first primary gradient field. Preferably, the second primary coil segment is located between the first and third primary coil segments.

Preferably, the shielding layer structure comprises a first, a second and a third shielding coil segment. Preferably, each coil segment of the shielding layer structure is arranged to generate an orthogonal gradient field opposite to the gradient field generated by the corresponding segment of the primary coil structure, thereby actively shielding the primary gradient field and reducing eddy currents in the magnet and in the DSV. Preferably, the shield structure is arranged around the primary coil structure and extends along substantially the entire axial length of the bore. Preferably, the diameter of each shield coil segment is greater than the diameter of any of the primary coil segments. The shielding layer structure and associated coils serve to shield the environment from the magnetic field generated by the magnetic coils.

Preferably, the first end of the bore has a diameter greater than 500mm to allow entry of the shoulder. Preferably, the diameter of the second end of the bore is about 100mm to 500 mm. This provides increased efficiency and linearity of the magnetic field gradient, in addition to the magnetic field (B)₀) While still providing access to the extremities.

Preferably, the shield coil is remote from the primary coil. Preferably, the shielding coil is frustoconical and/or cylindrical.

Preferably, the gradient coil system provides high efficiency, increased gradient strength and slew rate for head imaging and good linearity of the magnetic field gradient for short gradient lengths, and is suitable for head and extremity access.

Preferably, the polarity of the coils of the primary coil structure is opposite to the polarity of the corresponding coils of the shielding layer structure (i.e. they carry current in opposite directions).

Preferably, the system further comprises one or more shim bags (pockets). Preferably, the shim pockets are frusto-conical and/or cylindrical. Preferably, a shim portion is located in each shim pocket. Preferably, the shim portion comprises a ferrous or ferromagnetic material. Preferably, each primary coil segment of the gradient coil assembly has an associated shim pocket and a shim portion having a shape that conforms to the shape of the gradient coil. Preferably, the shimming portion passively shimmes the DSV to obtain a preferred field (B)₀) The level of homogeneity. Preferably, the shim portion is located between the primary coil structure and the shield structure. In some embodiments, the shim portion is located outside the shield structure. Preferably, the shim portion is located between the magnet and the gradient coil.

Preferably, the system further comprises one or more active magnetic shimming devices.

In a preferred embodiment, the DSV has dimensions of 300mm (x-) x300mm (y-) x300mm (z-). Preferably, the gradient coil assembly comprises two frusto-conical portions. Preferably, a first of the two frustoconical portions has an angle between 0 and 10 degrees with respect to the longitudinal axis of the gradient body and a second of the two frustoconical portions has an angle between 5 and 30 degrees with respect to the longitudinal axis of the gradient body.

More preferably, the angle of the first of the two frustoconical portions is 5 degrees and the angle of the second of the two frustoconical portions is 16 degrees with respect to the longitudinal axis of the gradient body.

Preferably, the diameter of the first end of the bore is between 500mm and 600mm and the diameter of the second end of the bore is between 100mm and 500 mm. More preferably, the diameter of the first end of the bore is 600mm and the diameter of the second end of the bore is 120 mm.

Preferably, the length of the second of the two frusto-conical portions is greater than the length of the first of the two frusto-conical portions to increase the efficiency and linearity of the magnetic field gradient. Preferably, the coil segments of the primary coil structure are arranged from the inner side (adjacent to the aperture) to the outer side (adjacent to the shield layer structure) in the following order: a Z primary coil, an X primary coil, and a Y primary coil. Preferably, the coil segments of the shield coil layer are arranged in the following order from the inside (adjacent to the primary coil layer) to the outside (adjacent to the magnet): an X shield coil, a Y shield coil and a Z shield coil. Preferably, the bore comprises two frustoconical portions coinciding with the two frustoconical portions of the gradient coil assembly.

In another preferred embodiment, the DSV has dimensions of 300mm (x-) x300mm (y-) x300mm (z-). Preferably, the gradient coil assembly comprises three stepped cylindrical portions, wherein each cylindrical portion has a different diameter. Preferably, the gradient coil assembly comprises a frusto-conical portion extending between two cylindrical portions. Preferably, the diameter of the first end of the bore is between 500mm and 600mm and the diameter of the second end of the bore is between 150mm and 300 mm. More preferably, the first end of the bore has a diameter of 560mm and the second end of the bore has a diameter of 210 mm. Preferably, the bore comprises a three-step cylindrical portion that coincides with a three-step cylindrical portion of the gradient coil assembly.

Preferably, the gradient coil system further comprises one or more Radio Frequency (RF) coils positioned between the gradient coil assembly and the bore. Preferably, the RF coil is frustoconical and/or cylindrical in shape conforming to the shape of the bore. Preferably, the RF coil is located on an inner surface of the gradient body surrounding the bore.

A gradient coil system adapted for use in an MRI system, the gradient coil system comprising:

a gradient coil assembly disposed around the bore, the gradient coil assembly having at least one frustoconical portion substantially conforming to the at least one frustoconical portion of the gradient body, the gradient coil assembly generating a gradient field for medical imaging in a diameter of a spherical volume (DSV).

Further features and advantages of the present invention will become apparent from the following detailed description.

Drawings

Preferred embodiments of the present invention will be described more fully hereinafter, by way of example only, with reference to the accompanying drawings, in which:

fig. 1 shows a schematic cross section of a frusto-conical gradient coil system in a magnet body according to a first embodiment of the invention;

FIG. 2 shows a schematic cross-section of a stepped gradient coil system having frusto-conical and cylindrical portions in accordance with a second embodiment of the invention;

FIG. 3 shows a schematic cross-section of a frustoconical gradient coil system having a frustoconical Radio Frequency (RF) coil in a magnet body according to a third embodiment of the invention;

FIG. 4 shows a schematic cross section of a stepped gradient coil system with a cylindrical RF coil according to a fourth embodiment of the present invention;

FIG. 5 shows a schematic cross-section of a frusto-conical gradient coil system having two frusto-conical sections according to a fifth embodiment of the invention;

FIG. 6 shows the axial (z) coil primary and shield patterns of the frustoconical gradient coil system shown in FIG. 5;

FIGS. 7a and 7b show transverse (x and y) coil primary and shield patterns of the frustoconical gradient coil system of FIG. 5;

FIG. 8 illustrates a portion of the stepped gradient coil system shown in FIG. 2;

FIG. 9 shows the axial (z) coil primary and shield patterns of the stepped gradient coil system shown in FIG. 8; and

FIG. 10 shows the transverse (x and y) coil primary and shield patterns of the stepped gradient coil system of FIG. 8.

Detailed Description

The present invention provides a gradient coil system having one or more tapered (or frustoconical) portions, and in some embodiments, one or more cylindrical portions. The invention facilitates signal encoding of magnetic resonance images and enhancement of static magnetic field homogeneity inside DSVs and is particularly useful for human head, limb and/or neonatal images and the like.

The gradient coil system has two ends, one with a large aperture to allow shoulder access and the other with a significantly smaller aperture to improve gradient efficiency and linearity of the magnetic field gradient, as well as shortening the gradient length.

The smaller gradient bore also allows extremity access for orthopedic imaging.

In some embodiments, frusto-conical or cylindrical passive/active shimming is included in the gradient system for shimming (i.e. homogenizing) the magnetic field (B) in the region of the Diameter (DSV) of the spherical volume₀)。

The gradient coil system includes a gradient coil assembly having a primary layer structure with a plurality of frustoconical and/or cylindrical portions, and a shield layer structure which may be frustoconical or cylindrical depending on the magnet configuration.

The main layer structure comprises three coil segments which generate three orthogonal linear gradient fields in the DSV region, one of which is called axial coil which generates a gradient field along the longitudinal axis (z-axis) and the other two are called transverse coils which are rotated 90 degrees with respect to each other.

The shielding layer structure further comprises three coil segments generating three orthogonal gradient fields opposite to the gradient fields generated by the associated primary coil in order to actively shield the primary gradient fields and reduce magnet-neutral DSV

Of the vortex flow.

In the primary coil, the diameter of the wide bore is large enough to allow shoulder access, preferably greater than 500mm, while the diameter of the narrow bore is envisaged to be small enough to have the high efficiency and linearity of the magnetic field gradient and shorter gradient length (relative to a pure cylindrical system) and still provide access to the limbs. In this regard, it is envisaged that the diameter of the narrow bore will be between 100mm and 500 mm.

To improve or optimize shielding performance, the shield coil is as far away from the primary coil as possible. As a result, the shield coil may be cylindrical to the cylindrical magnet bore.

Turning now to fig. 1, a frustoconical gradient coil system 01 for a Magnetic Resonance Imaging (MRI) system is shown. The frusto-conical gradient coil system 01 comprises a frusto-conical gradient body 102 inserted into a chamber (i.e. an axial opening) of a magnet 101 having a Diameter (DSV)111 of an associated spherical volume.

The gradient body 102 defines a substantially frustoconical bore 104 that extends axially along the length of the gradient body 102. As can be seen, the aperture 104 includes a first opening 104a and a second opening 104 b.

The first opening 104a is intended to allow access to the shoulder of the patient for imaging and must therefore be sized accordingly.

As mentioned briefly above, it is desirable that the first opening 104a (i.e., the larger opening) be appropriately sized to allow access to the shoulder of the patient. Thus, it is contemplated that the first opening 104a will have a diameter D11 of no less than 500mm, while the diameter D13 of the second opening 104b (i.e., the smaller opening) should be small enough to provide high efficiency and gradient linearity of the gradient field and shorter gradient length, and still provide access to the extremities (e.g., hands and arms).

The gradient coil assembly 120 is located within the gradient body 102. The gradient coil assembly 120 comprises three

frustoconical portions

121a, 121b, 121c comprising a primary coil 121, and three frustoconical portions of a

shield coil

122a, 122b, 122c comprising a shield coil 122. There is also a frusto-conical passive shimming arrangement 123 comprising shim pockets housing one or more shim portions. The shimming section provides passive shimming of the magnetic field and comprises a ferromagnetic material.

Each

primary coil portion

121a, 121b or 121c has a different angle θ 1a, θ 1b, θ 1c and a different average diameter D11, D12, D13 with respect to the longitudinal axis 105 of the magnet 101. However, the angles θ 2a, θ 2b, θ 2c of each of the

shield coil portions

122a, 122b, 122c may be the same or different. Advantageously, the various configurations provided by the different angles of the primary coil portions 121a-c and the shield coil portions 122a-c produce the best and desired magnetic field in the DSV and minimize stray magnetic fields.

As described with respect to the first opening 104a of the bore 104, the diameter of the innermost primary coil section primary coil 121a is spaced wide enough to allow access by the patient's shoulders. Therefore, like the first opening 104a, it is preferable that the primary coil 121a has a maximum diameter of not less than 500mm, but it is conceivable that the primary coil 121a may be any size.

Each of the primary coil 121 and the secondary coil 122 comprises three coil segments (z, x and y coils), respectively, which generate three orthogonal gradient fields in three orthogonal z, x and y axes.

The direction of the current in the shield coil 122 is opposite to the direction of the current in the corresponding primary coil 121. This will be more clearly illustrated in the following figures relating to further embodiments.

As shown, the primary coil 121 and the shield coil 122 are spaced sufficiently apart such that the passive shim pocket 123 extends therebetween from a narrow end of the gradient body 102 (adjacent the second opening 104b) into a wide portion of the gradient body 102 (adjacent the first opening 104 a).

By positioning the passive shim pockets 123 between the primary coils 121 and the shield coils 122, shim portions may be positioned in the shim pockets 123 to shim the magnet DSV 111 to a desired level of field uniformity.

The gradient coil system 02 shown in fig. 2 comprises a three-step gradient body 202 inserted into a chamber (i.e., an axial opening) of a magnet 201 having a Diameter (DSV)211 of an associated spherical volume.

The gradient body 202 defines a stepped bore 204 extending axially along the length of the gradient body 202. As can be seen, the aperture 204 includes a first opening 204a and a second opening 204 b.

The first opening 204a is intended to allow access to the shoulder of the patient for imaging and must therefore be sized accordingly. As described above with respect to system 01, it is contemplated that the diameter of the first opening 204a (i.e., the larger opening) will be no less than 500mm, while the second opening 204b (i.e., the smaller opening) should be small enough to provide high efficiency and gradient linearity of the gradient field and shorter gradient length, and still provide access to extremities (e.g., hands and arms).

Located within the gradient body 202 are three stepped cylindrical

primary coil portions

221a, 221b, 221c and a frustoconical primary coil portion 221d comprising the primary coil 221, and three stepped cylindrical

shielding coil portions

222a, 222b, 222c and a frustoconical shielding coil portion 222d comprising the shielding coil 222.

In addition, there are two stepped cylindrical

passive shimming devices

223a and 223 b. Each

shim device

223a, 223b is substantially similar to the shim device 123 described above having shim pockets housing one or more shim pieces of ferromagnetic material.

As can be seen, the primary and

shield coil portions

221a, 222a and the primary and

shield coil portions

221c, 222c are radially separated and spaced around the aperture 204 to provide space for the

shimming devices

223a, 223b to be respectively located therebetween. As described above, the shim pieces located in the shim pockets of the

shim devices

223a and 223b work together to shim the magnet DSV 211 to a desired level of field uniformity (e.g., 10ppm in a 300mm DSV).

Each step of the

primary coil portions

221a, 221b, 221c, 221D has portions with different average diameters D21, D22, D23, D24, respectively.

The diameter D21 of portion 221a should be large enough to allow entry of the patient's shoulders, and the diameter D24 of portion 221c should be large enough to allow entry of the extremities.

Although the diameter D21-D24 of each step of the primary coil portions 221a-c is different, the diameter of each shield coil portion 222a-c may alternatively be different or substantially equal.

Each portion of the gradient coil 220, including the primary coil 221 and the shield coil 222, includes three coil segments, referred to as z, x, and y coils, which generate three orthogonal gradient fields in three orthogonal z, x, and y axes.

The direction of the current in the shield coils 222a-c is opposite to the direction of the current in the respective primary coils 221 a-c.

Referring to fig. 3, there is another embodiment of the invention in the form of a frusto-conical gradient coil system 03. The frustoconical gradient coil system 03 is substantially similar to the frustoconical gradient coil system 01 described above. However, the primary coil portion 321a is parallel to the longitudinal axis 305 of the magnet 301, as opposed to the primary coil portion 121a of the angled system 01.

The frustoconical gradient coil system 03 includes a frustoconical gradient body 302 inserted into a chamber (i.e., an axial opening) of a magnet 301 having a Diameter (DSV)311 of an associated spherical volume.

The gradient body 302 defines a substantially frustoconical bore 304 that extends axially along the length of the gradient body 302. As can be seen, the aperture 304 includes a first opening 304a and a second opening 304 b.

The first opening 304a is intended to allow access to the shoulder of the patient for imaging and must therefore be sized accordingly.

As mentioned briefly above, it is desirable that the first opening 304a (i.e., the larger opening) be appropriately sized to allow access to the shoulder of the patient. Thus, it is contemplated that the first opening 304a will have a diameter D31 of no less than 500mm, while the diameter D33 of the second opening 304b (i.e., the smaller opening) should be small enough to provide high efficiency and gradient linearity of the gradient field and shorter gradient length, and still provide access to the extremities (e.g., hands and arms).

The gradient coil assembly 320 is located within the gradient body 302. The gradient coil assembly 320 comprises three

frustoconical portions

321a, 321b, 321c comprising the primary coil 321 and three frustoconical portions comprising the shield coils 322a, 322b, 322c of the shield coil 322. There is also a frusto-conical passive shimming device 323 comprising a shim pocket housing one or more shim portions. The shimming portion provides passive shimming of the magnetic field and comprises a ferromagnetic material.

The

primary coil portions

322c, 322b each have a different respective angle θ 3a, θ 3b relative to the longitudinal axis 305 of the magnet 301, and a different average diameter D31, D32, D33. However, the angles θ 3c, θ 3d of the respective

shield coil portions

322c, 322b may be the same or different. Advantageously, the various configurations provided by the different angles of the primary coil portions 321a-c and the shield coil portions 322a-c produce the best and desired magnetic field in the DSV and minimize stray magnetic fields.

As described with respect to the first opening 304a of the bore 304, the diameter of the innermost primary coil portion primary coil 321a is spaced wide enough to allow entry of the patient's shoulder. Therefore, like the first opening 304a, it is preferable that the primary coil 321a has a maximum diameter of not less than 500mm, but it is conceivable that the primary coil 321a may be any size.

Each of the primary coil 321 and the shield coil 322 comprises three coil segments (z, x and y coils), respectively, which generate three orthogonal gradient fields in three orthogonal z, x and y axes.

The direction of the current in the shield coil 322 is opposite to the direction of the current in the corresponding primary coil 321. This will be more clearly illustrated in the following figures relating to further embodiments.

As shown, the primary coil 321 and the shield coil 322 are spaced sufficiently apart such that the passive shim pocket 323 extends therebetween from a narrow end of the gradient body 302 (adjacent the second opening 304b) to a wide portion of the gradient body 102 (adjacent the first opening 304 a).

By positioning the passive shim pocket 323 between the primary coil 321 and the shield coil 322, shims may be positioned in the shim pocket 323 to shim the magnet DSV 311 to a desired level of field uniformity.

In addition, the frustoconical gradient coil system 03 also includes a frustoconical Radio Frequency (RF) coil 303 located on an inner surface of a bore 304 extending through the gradient body 302. It should be understood that the RF coil may be a volume coil, a surface coil, or a combination of both.

These frusto-conical RF coils 303 are configured as receivers adapted to receive radio frequency signals of the magnetic resonance imaging system.

Similar to system 01, each of the primary coils 321 and the shield coils 322 respectively includes three coil segments (z, x, and y coils) that generate three orthogonal gradient fields in three orthogonal z, x, and y axes. The direction of the current in the shield coils 322a-c is opposite to the direction of the current in the respective primary coils 321 a-c.

Another embodiment of the invention in the form of a stepped gradient coil system 04 is shown in fig. 4.

The stepped gradient coil system 04 is substantially similar to the gradient coil system 02 described above and shown in FIG. 2. However, the stepped gradient coil system 04 also includes a frustoconical Radio Frequency (RF) coil 203 on the inner surface of the bore 204 extending through the gradient body 202.

These frusto-conical RF coils 203 are configured as receivers adapted to receive radio frequency signals of the magnetic resonance imaging system.

As with the embodiments described herein, the direction of current flow in the shield coils 222a-c is opposite to the direction of current flow in the respective primary coils 221 a-c.

In a particularly preferred embodiment shown in FIG. 5, the DSV 511 of a frusto-conical gradient coil system 05, substantially similar to the gradient coil system 01, has dimensions of 300mm (x-) x300mm (y-) x300mm (z-).

The gradient coil system 05 comprises two frustoconical portions: a wide portion 502a and a narrow portion 502b formed in the gradient body 502.

The gradient coil assembly 520 comprises two frustoconical portions 521a containing primary coils 521

And 521b, and two frustoconical portions of shield coils 522a and 522b that contain shield coil 522.

With respect to the axial direction of the gradient (represented by line 505), the angle θ 5a of the wide portion 502a is equal to 5 degrees, and the angle θ 5b of the narrow portion 502b is equal to 16 degrees.

The maximum radius R51 of the widest gradient aperture 504a is 300mm (equivalent to a diameter of 600 mm) and the minimum radius R52 of the narrowest aperture 504b is 60mm (equivalent to a diameter of 120 mm).

In this particularly preferred embodiment, the length of narrow portion 502b is significantly longer than the length of wide portion 502a to increase the efficiency and linearity of the magnetic field of system 05.

As can be seen in the figure shown, the primary coil 521 and the shielding coil 522 are arranged in order from the inside to the outside: z primary coil, X primary coil, Y primary coil, X shield coil, Y

The shield coil, then the Z shield coil.

A shimming device 523, substantially similar to the shimming device 123 described above, is located between the primary coil 521 and the shield coil 522.

The direction of the current in the shield coil 522 (shown in fig. 6, 7a and 7 b) is opposite to the direction of the current in the corresponding primary coil 521.

In use, the above-described embodiments provide high efficiency and linearity of the magnetic field gradient. In addition, the system 05 provides adequate access to the patient's shoulders using a small angle of 5 degrees at the first end and a large angle of 16 degrees at the second end.

In another particularly preferred embodiment according to fig. 8, a three-step gradient coil system 06, which is substantially similar to the three-step gradient coil system 02 and uses a number of similarly numbered components, has a DSV 611 with dimensions 300mm (x-) x300mm (y-) x300mm (z-).

As described above, the gradient coil assembly 220 includes three stepped

cylindrical portions

202a, 202b, 202c and a frustoconical portion 202d located between the

cylindrical portions

202b and 202 c. Each of the sections 202a-d has a different radius R21, R22, R23, R24, respectively.

The radius R21 of the widest gradient orifice 204a is 280mm (corresponding to a diameter of 560 mm) and the radius R24 of the narrowest orifice 204b is 105mm (corresponding to a diameter of 210 mm).

The primary coil 221 and the shield coil 222 are arranged in order from the inside (adjacent to the bore 204) to the outside (moving radially outwards from the bore 204): a Z primary coil, an X primary coil, a Y primary coil, an X shield coil, a Y shield coil, and then a Z shield coil, as can be clearly seen in the figure.

The direction of the current in the shield coils 222 (shown in fig. 9 and 10) is opposite to the direction of the current in the respective primary coils 221.

In use, the system 06 described above provides access to the shoulders of the patient at the widest gradient bore 204a and to the extremities of the patient at the narrowest bore 204b, while maintaining high efficiency and linearity of the magnetic field.

In some embodiments, the larger opening provided by the frustoconical bore (relative to a cylindrical system) may reduce the overall length of the magnet required in the system.

Although the embodiments described herein include passive magnetic shimming devices, alternative embodiments may also include active magnetic shimming devices.

Embodiments of the invention described herein aim to provide high efficiency, high gradient strength and hence improved slew rate for head imaging, improved linearity for shortened gradient length and a system suitable for head and extremity access. As will be appreciated, increased gradient strength and high slew rates may be particularly important (and often necessary) for cardiac and head (i.e., brain) imaging.

In addition, embodiments of the present invention have portions of multiple frusto-conical gradient coils with different angles and diameters, including shimming to provide efficient and high-speed spatial encoding of Nuclear Magnetic Resonance (NMR) signals, and for shoulder and extremity easy-entry MRI systems.

Embodiments of the present invention having primary coil portions positioned at different angles produce an optimal magnetic field in the DSV and minimize stray magnetic fields.

Advantageously, the frustoconical nature of the components described above with respect to the various embodiments of the invention allows the DSV to be displaced from the center of the bore. This is in contrast to typical cylindrical design DSVs, which typically have a DSV located in the center. In the embodiments of the invention described herein, this allows for easier positioning of the patient for head imaging (from the large end) and extremity imaging (from the narrow end) within the imaging zone.

In another advantage, the frustoconical design of embodiments of the present invention allows for the use of smaller magnets, thereby reducing power consumption and overall costs, including costs associated with materials and installation. In addition, the smaller size of the scanner requires less installation space, thereby minimizing the footprint of the scanner.

This can be a critical consideration when installing scanners in modern hospitals.

In this specification, adjectives such as first and second, left and right, top and bottom, and the like may be used solely to distinguish one element or action from another element or action without necessarily requiring or implying any actual such relationship or order. Where the context permits, references to integers or components or steps (or the like) should not be construed as limited to only one of the integers, components or steps, but may be one or more of the integers, components or steps, and the like.

The foregoing detailed description of various embodiments of the invention is provided to those skilled in the art for the purpose of illustration. It is not intended to be exhaustive or to limit the invention to the precise embodiment disclosed. Many alternatives and variations of the present invention, as described above, will be apparent to those skilled in the art in light of the above teachings. Accordingly, while some alternative embodiments have been discussed in detail, other embodiments will be apparent to or relatively easily developed by those of ordinary skill in the art. The present invention is intended to embrace all such alternatives, modifications and variances which have been discussed herein, and other embodiments which fall within the spirit and scope of the above described invention.

In this specification, the terms "comprises," "comprising," "includes" or similar terms are intended to mean a non-exclusive inclusion, such that a method, system or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed.

Throughout the specification and claims (if present), unless the context requires otherwise, the term "substantially" or "about" will be understood to not be limited to the specific value or range defined by the term.

Claims

1. A gradient coil system adapted for use in an MRI system, the gradient coil system comprising:

2. The gradient coil system of claim 1, wherein the gradient body comprises one or more cylindrical portions arranged around the bore, wherein at least one of the one or more cylindrical portions abuts one or more of the at least one frustoconical portions.

3. The gradient coil system of claim 2, wherein a first cylindrical portion having a diameter adjoins a second cylindrical portion having a diameter, wherein the diameter of the first cylindrical portion is greater than the diameter of the second cylindrical portion.

4. The gradient coil system of claim 1, wherein the gradient body comprises a plurality of frustoconical portions and/or a plurality of cylindrical portions defining a stepped diameter bore.

5. The gradient coil system of any of the preceding claims, wherein the gradient coil assembly comprises a primary coil structure having at least one substantially frustoconical portion.

6. The gradient coil system of claim 5, wherein the gradient coil assembly further comprises a shielding layer structure having at least one substantially frustoconical portion corresponding to the at least one frustoconical portion of the primary coil structure.

7. The gradient coil system of claim 6, wherein the primary coil structure comprises first, second and third primary coil segments generating three orthogonal linear primary gradient fields in the DSV region, wherein the first primary coil segment generates a gradient field along a z-axis, the second primary coil segment generates a gradient field along an x-axis, and the third primary coil segment generates a gradient field along a y-axis.

8. The gradient coil system of claim 7, wherein the first primary coil segment comprises an axial coil that generates a first primary gradient field along a longitudinal axis corresponding to the z-axis, and wherein the second and third primary coil segments each comprise a transverse coil rotated 90 degrees relative to each other, thereby generating second and third primary gradient fields that are orthogonal to each other and to the first primary gradient field.

9. The gradient coil system of any of claims 6-8, wherein the shielding layer structure comprises first, second and third shielding coil segments, and wherein each of the coil segments of the shielding layer structure is arranged to generate an orthogonal gradient field that opposes the gradient field generated by the corresponding segment of the primary coil structure, thereby actively shielding the primary gradient field and reducing eddy currents in a magnet and in the DSV, wherein the first shielding coil segment generates a gradient field along the x-axis, the second primary coil segment generates a gradient field along the y-axis, and the third primary coil segment generates a gradient field along the z-axis.

10. The gradient coil system of any of claims 6-9, wherein the shielding layer structure is provided around the primary coil structure and extends substantially along an axial length of the bore, and wherein a diameter of each of the shielding coil segments is greater than a diameter of any of the primary coil segments.

11. The gradient coil system of any of claims 6-10, wherein the shield coil segments are frustoconical and/or cylindrical in shape conforming to a shape of the corresponding primary coil segment.

12. The gradient coil system of any of claims 6-11, wherein a polarity of the coils of the primary coil structure is opposite a polarity of the respective coils of the shielding layer structure.

13. The gradient coil system of any preceding claim, wherein the system further comprises one or more shim pockets and a shim portion located in each shim pocket, whereby, in use, the shim portions passively shim the DSV to achieve a predetermined field (B ™)₀) The level of homogeneity.

14. The gradient coil system of claim 13, wherein the shim portions comprise ferrous or ferromagnetic material.

15. The gradient coil system of claim 13 or 14, wherein each primary coil segment of the gradient coil assembly has an associated shim pocket and a shim portion having a shape that conforms to the shape of the primary coil segment.

16. The gradient coil system of any of claims 13-15, wherein the shim portions are located between the primary coil structure and the shield layer structure.

17. The gradient coil system of any of claims 13-15, wherein the shim portions are located outside of the shielding layer structure.

18. The gradient coil system of any of claims 13-15, wherein the shim portion is located between a magnet chamber surrounding the gradient coil assembly and the gradient coil assembly.

19. The gradient coil system of any of claims 13-15, wherein the system further comprises one or more active magnetic shimming devices.

20. The gradient coil system of any of the preceding claims, wherein the gradient coil assembly comprises two frustoconical portions.

21. The gradient coil system of claim 20, wherein, relative to a longitudinal axis of the gradient body, an angle of a first of the two frustoconical portions is between 0 degrees and 10 degrees, and an angle of a second of the two frustoconical portions is between 5 degrees and 30 degrees.

22. The gradient coil system of claim 21, wherein the angle of the first of the two frustoconical portions is 5 degrees and the angle of the second of the two frustoconical portions is 16 degrees relative to the longitudinal axis of the gradient body.

23. The gradient coil system of claim 22, wherein the length of the second of the two frustoconical portions is greater than the length of the first of the two frustoconical portions to increase the efficiency and linearity of the magnetic field gradient.

24. The gradient coil system of claim 1, wherein the coil segments of the primary coil structure are arranged in the following order from adjacent the bore to adjacent the shielding layer structure: the first primary coil segment, the second primary coil segment, and the third primary coil segment.

25. The gradient coil system of claim 24, wherein the coil segments of the shielding layer structure are arranged in the following order from adjacent the primary coil layer to adjacent a magnet surrounding the gradient coil assembly: the first shield coil segment, the second shield coil segment, and the third shield coil segment.

26. The gradient coil system of claim 20, wherein the bore comprises two frustoconical portions corresponding to the two frustoconical portions of the gradient coil assembly.

27. The gradient coil system of claim 1, wherein the gradient coil assembly comprises three stepped cylindrical portions, wherein each cylindrical portion has a different diameter, and wherein the bore comprises three stepped cylindrical portions that coincide with the three stepped cylindrical portions of the gradient coil assembly.

28. The gradient coil system of claim 27, wherein the gradient coil assembly comprises a frustoconical portion extending between two of the cylindrical portions.

29. The gradient coil system of claim 1, wherein the gradient body further comprises a single frustoconical portion extending along a length of the bore between the first end and the second end.

30. The gradient coil system of any of the preceding claims, wherein the gradient coil system further comprises one or more Radio Frequency (RF) coils located between the gradient coil assembly and the bore.

31. The gradient coil system of claim 30, wherein the RF coil is frustoconical and/or cylindrical in shape conforming to a shape of the bore.

32. The gradient coil system of claim 30 or 31, wherein the RF coil is located on an inner surface of the gradient body surrounding the bore.

33. The gradient coil system of any of the preceding claims, wherein the diameter of the first end of the bore is greater than 500mm to allow shoulder access and the diameter of the second end of the bore is approximately 100-500 mm.

34. The gradient coil system of claim 33, wherein the diameter of the first end of the bore is between 500mm and 600mm and the diameter of the second end of the bore is between 100mm and 500 mm.

35. The gradient coil system of claim 34, wherein the diameter of the first end of the bore is 600mm and the diameter of the second end of the bore is 120 mm.

36. The gradient coil system of claim 33, wherein the diameter of the first end of the bore is between 500mm and 600mm and the diameter of the second end of the bore is between 150mm and 300 mm.

37. The gradient coil system of claim 36, wherein the diameter of the first end of the bore is 560mm and the diameter of the second end of the bore is 210 mm.

38. The gradient coil system of any of the preceding claims, wherein the DSV has dimensions of 300mm (x-) x300mm (y-) x300mm (z-).

39. The gradient coil system of any of the preceding claims, wherein the gradient body is located within a chamber of a magnet.